In recent years, to address environmental problems and the like, weight reduction and enhanced strength are being sought with respect to members used in automobiles, industrial machinery, buildings and the like. In particular, tensile strength of 1000 MPa or more is being demanded with respect to bolts for automobiles, such as engine cylinder head bolts and connecting rod bolts.
However, if a bolt has high tensile strength of 1000 MPa or more, the susceptibility to hydrogen embrittlement increases, and hydrogen embrittlement resistance (delayed fracture) characteristics are lowered. SCM steel (JIS Standard) that contains a large amount of an alloying element such as Mo, and alloy steel that contains an expensive alloying element such as V, are used as the starting material for such high-strength bolts. These alloy steels are manufactured into wire rods, and are further subjected to wire drawing and cold forging to be manufactured into bolts.
In the case of using the aforementioned alloy steels as bolts, hydrogen embrittlement resistance characteristics are enhanced. However, because these alloy steels contain alloying elements in large amounts, it leads to an increase in the cost of the steel material. Further, in recent years, the prices of alloying elements have risen sharply, and the supply-and-demand environment easily fluctuates. Therefore, there is a need for a bolt, in which enhanced strength and excellent hydrogen embrittlement resistance characteristics can be realized while reducing or omitting these alloying elements and keeping down the cost of the steel material.
To keep down the cost of the steel material, it is sufficient to reduce the amount of alloying elements, such as Mo and V, contained in the steel. If the amount of alloying elements is reduced, the hardenability of the steel material decreases, and when the steel material is hot rolled to produce wire rods, the formation of a hard microstructure such as bainite can be suppressed. Therefore, a softening heat treatment can be omitted or simplified, and the production cost is reduced. However, it causes difficulty in providing the bolt with high strength, and the hydrogen embrittlement resistance characteristics also decrease.
Therefore, studies are being conducted with respect to high-strength bolts that contain boron (B) instead of alloying elements, such as Mo and V. Similarly to alloying elements such as Mo and V, B enhances the hardenability of steel. However, if B-containing steel is used for a high-strength bolt whose tensile strength is 1000 MPa or more, in some cases, the hydrogen embrittlement resistance characteristics will be low.
Bolts for overcoming this problem are proposed in Japanese Patent Application Publication No. 2012-162798 (Patent Literature 1), Japanese Patent Application Publication No. 11-293401 (Patent Literature 2), Japanese Patent Application Publication No. 10-53834 (Patent Literature 3) and Japanese Patent Application Publication No. 2008-156678 (Patent Literature 4). The bolts proposed in these Patent Literatures contain boron to thereby increase hardenability, strengthen grain boundaries to increase the strength, and also enhance hydrogen embrittlement resistance characteristics.
Specifically, the steel for a high-strength bolt that is disclosed in Patent Literature 1 contains, in mass %, C: 0.20 to less than 0.40%, Si: 0.20 to 1.50%, Mn: 0.30 to 2.0%, P: 0.03% or less (not including 0%), S: 0.03% or less (not including 0%), Ni: 0.05 to 1.0%, Cr: 0.01 to 1.50%, Cu: 1.0% or less (including 0%), Al: 0.01 to 0.10%, Ti: 0.01 to 0.1%, B: 0.0003 to 0.0050% and N: 0.002 to 0.010%, in which one or more types selected from the group consisting of Cu, Ni and Cr are contained in a total amount of 0.10 to 3.00/o, with the balance being Fe and unavoidable impurities. In the steel, a ratio ([Si]/[C]) between the Si content [Si] and the C content [C] is 1.0 or more, and the steel has a ferrite and pearlite microstructure. It is described in Patent Literature 1 that by this means a B-added high-strength bolt that is excellent in delayed fracture resistance can be obtained.
In the B-added high-strength bolt disclosed in Patent Literature 1, the Si content is made higher than the C content to increase the strength of the matrix by means of Si and to improve the delayed fracture resistance. However, because Ni that is an expensive element is contained as an essential element, the cost of the steel material is high.
The steel for a bolt that is disclosed in Patent Literature 2 contains, in mass %, C: 0.10 to 0.45%, B: 0.0003 to 0.0050%, Ti: 0.01 to 0.1% and N: 0.0025 to 0.010%, and furthermore, as other constituents, contains Si: 0.03 to 0.5%, Mn: 0.3 to 1.5% and Al: 0.01 to 0.10%, with the balance being Fe and unavoidable impurities. The steel also satisfies at least one of the conditions of the following (1) and (2). (1) The Ti amount contained in precipitates having a grain size of more than 0.1 μm extracted by the extraction residue method is 60% or more of the total Ti amount contained in the steel material. (2) The average number of Ti-based precipitates having a grain size of 0.01 to 0.2 μm observed by electron microscope observation according to the extraction replica method is from 10 to 500 in an observation visual field of 25 μm2. It is described in Patent Literature 2 that by this means a bolt made of B-containing steel that is excellent in cold workability and delayed fracture resistance can be obtained.
However, in the bolt disclosed in Patent Literature 2, the Si content is low, and the mass ratio between Si and Mn is less than 1.0. Therefore, it is difficult to control inclusions, and the hydrogen embrittlement resistance characteristics are low in some cases.
A steel for a high-strength bolt that is disclosed in Patent Literature 3 contains, in mass %, B: 0.0008 to 0.004%, C: 0.4% or less (not including 0%), Ti: 0.025 to 0.06%, and N: 0.006% or less (not including 0%), with the balance being Fe and unavoidable impurities. In this steel, the relation between a ferrite grain size FGc and Ti compounds excluding TiN during hot rolling satisfies the expression: [amount of Ti compounds excluding TiN/FGc1/2]×1000≥3. It is described in Patent Literature 3 that, as a result, the austenite grain size number is 5 or more, and a high-strength bolt having a tensile strength that is more than 785 N/mm2 can be obtained.
However, in the high-strength bolt disclosed in Patent Literature 3, when there is a high Mn content and a low Cr content, the hydrogen embrittlement resistance characteristics are low in some cases.
A steel for a high-strength bolt disclosed in Patent Literature 4 contains, in mass %, C: more than 0.15% to 0.30% or less, Si: 1.0% or less, Mn: 1.5% or less, Ti: 0.1% or less, Mo: 0.3% or more to 0.5% or less and B: 0.0005% or more to 0.01% or less, with the balance being Fe and impurities. The steel is quenched, and thereafter subjected to tempering at 100 to 400° C., and the steel microstructure becomes a microstructure in which the average prior-austenite grain size after quenching is 10 μm or less. It is described in Patent Literature 4 that, by this means, a high-strength bolt that has the bolt strength range from approximately 1200 to 1600 MPa and has excellent delayed fracture resistance characteristics and corrosion resistance can be obtained.
However, because the bolt disclosed in Patent Literature 4 contains 0.3 to 0.5% by mass of Mo, the hardenability is too high. Consequently, it is necessary to carry out a softening heat treatment for an extended period of time before performing wire drawing and cold forging. In this case, the production cost increases significantly in some instances.